The combustion of biomass and other energy containing organic matter with a high water content typified by bark, lignite, wood, garbage, coal slurries and waste products, some of which also contain a high percentage of alkali metals, sulfur, silicon oxides, presents a difficult problem if the combustion apparatus is to burn the material efficiently, produce acceptable levels of emissions in the effluent gas and permit adequate "turndown" of the combustion process to match the thermal energy needs of a domestic or industrial energy user.
Wood residue presents a particular problem with regard to its disposal at sawmills, paper mills, and other types of wood processing plants. The residue often contains a large amount of moisture and is known to produce particulate in the emissions in significant amounts when burned in conventional equipment such as Teepee burners. Thus, it has been a problem to merely dispose of wood residue and meet the National environmental standards by burning even if one is not concerned with harnessing the energy produced by such burning.
It is the primary objective of this invention to provide a solid fuel combustor which has high burning efficiency enabling the conversion of the energy contained therein to a usable form suitable for the generation of steam, electric power, or simply the heating of air for space heating or for the dehydration of processed materials.
It is a further objective of this invention to combust solid fuels containing high percentages of impurities and water in an efficient manner so that a large portion of the energy content of the fuel is effectively employed and not permitted to escape with the effluent gas to the atmosphere as combustible vapors and unburned hydrocarbons.
It is a further objective of this invention to combust industrial process plant residuals in a manner that prevents the escape of small quantities of inorganic and unburned particulate matter of the submicron size, in particular, to escape into the atmosphere with the effluent gas.
It is a further objective of this invention to maintain a stable and efficient combustion reaction with a high turndown ratio; i.e., the ratio of the energy release at maximum rating to the energy release at minimum output with stable combustion without the use of ancillary fuels such as natural gas or fuel oil so that when the demand for energy is low, the wood residue is not consumed unnecessarily.
It is a further object of this invention to permit close control over the temperature of the products of combustion leaving the combustor to avoid damaging equipment downstream of the combustor by using essentially inert diluents which can be introduced into combustion process such as water, air, recirculated products of combustion, or any other suitable gas.
According to the invention a solid fuel combustor has a primary combustion chamber with means for introduction therein of primary air and a fuel entrance port for filling the chamber with particulate fuel. Preferably the fuel entrance port can be sealed once the chamber is charged with solid fuel. A support grate underlies the primary combustion chamber for carrying fuel. A secondary combustion chamber preferably lies below the primary combustion chamber and there is a means for introducing secondary air to the combustor to cause burning in the secondary combustion chamber after primary burning in the primary combustion chamber. The secondary combustion chamber has an ash removal port and screen means to screen ash from flow of gaseous combustion products. Preferably the screen means is in the form of a refractory brick wall having a plurality of exit slots so that gas passes out of the chamber in one direction and ash drops through the chamber in a downward direction. Tertiary air or exhaust diluent gas can be introduced into a manifold area to polish the combustion process from the secondary chamber and/or modify its temperature to a low enough temperature to avoid damage to equipment further on in the gas path.
A manifold is positioned to receive gas flowing from the secondary combustion chamber and preferably surrounds the combustion chamber. A heat exchanger is coupled to the manifold for extracting heat from the gaseous output of the manifold which heat is used to heat primary and secondary air.
In the preferred embodiment, a grate has a series of parallel tubes for supporting solid particulate fuel and is mounted to allow oscillation. The oscillation can be used in mixing the gases from the primary chamber with secondary air. The oscillation is primarily used to dislodge ash and unburned solid fuel which might otherwise clog spaces between the grate tubes and thus increase the pressure drop of gases flowing through the tubes, to an undesirable level. The preferred grate is formed of parallel generally cylindrical ceramic elements having secondary gas flow passages formed by tubes or pipes therein for passing secondary air to a combustion gas flow passing through slots between the cylindrical elements. The secondary gas flow can also act to cool the grate and thus prevent heat damage to it.
According to the method of this invention solid particulate fuel such as waste fuel is burned efficiently by introducing the fuel in particulate form into a primary combustion chamber with the fuel supported by a grate under which is a secondary combustion chamber. Drying and pyrolysis takes place in the primary combustion chamber. Primary combustion air is passed into the primary chamber to create a gaseous combustion product flow through the grate to an underlying secondary combustion chamber area. Secondary air is added to the combustion product flow to cause secondary combustion in the secondary combustion chamber whereupon the gaseous flow is impinged against an insulating screen so that ash is removed from the flow with the gaseous flow passing to a manifold area and thence to a heat exchanger. At least some of the primary combustion air is heated in the heat exchanger utilizing the heat derived from the gaseous exhaust flow.
It is an important feature of this invention that the combustor is so efficient over a large turndown rate that it provides for sufficiently complete combustion to facilitate satisfaction of Environmental Protection Agency standards for air pollution and is also efficient from a thermo-dynamic standpoint when used not only for waste fuel destruction but also to extract energy for useful purposes such as running of a boiler, turbine or the like. It is another feature of this invention that wet fuels can be efficiently burned.
The use of a multi-stage burning process enables one to keep the temperature high enough to complete combustion but low enough to minimize NOX formation. In effect, the fuel is burned to CO and partially burned pyrolysis products above the grate and to CO.sub.2 and water vapor under the grate. In some cases the tertiary air added finishes combustion if not previously finished during the secondary combustion stage.
Still another feature of the invention relates to the use of a ceramic heat exchanger as a recuperator to enable efficient recuperation of heat for use in the primary or secondary combustion air flow to obtain temperatures of 1000.degree. F. and sometimes higher. This helps to enable burning of wet fuel substantially as completely as any other fuel.